If you get an extension to pay your tax bill jointly, can you change your mind and file seperately before Oct. 15?

Assuming the Form 4868 had both of your names and social security numbers on it, there seems to be nothing on the Form 4868 or the filing instructions preventing you from filing separately.
However, you need to be sure that your $800 refund is the result of you filing status as "married filing separately" and not as single. You must file as married filing separately.

Hope this helps you.

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Mr. Fromm is licensed to practice law in PA. The response herein is not legal advice and does not create an attorney/ client relationship. The response is in the form of legal education and is intended to provide general information about the matter within the question. Oftentimes the question does not include significant and important facts and timelines that if known could significantly change the reply unsuitable. Mr. Fromm strongly advises the questioner to confer with an attorney in their state in order to ensure proper advice is received.